Current limiting is the practice of imposing an upper limit on the current that may be delivered to a load. The typical purpose of current limiting is to protect the circuit up or downstream from harmful effects due to, for example, a short circuit. In load switch applications used in power sources and adapters, current may be limited below a load switch setting. Load switch applications include driving a power line of a universal serial bus (USB) connector to various peripheral devices. Examples of load switch devices include current limited load switch devices produced by Advanced Analogic Technologies, Inc. (Sunnyvale, Calif.) as integrated circuits (ICs) designed to protect external power ports and to extend battery life in portable electronic products. Such load switch devices operate with an integrated current limiting circuit that protects, for example, the input supply against large changes in load current which could otherwise cause the supply to fall out of regulation.
As current limited devices, load switches are able to draw current up to the load switch setting. If the current exceeds the load switch setting, the current limiting circuit in the load switch limits the current flowing through the load switch. Typically, a resistor (either external or internal to the IC) is used to set the load switch current limits. Typically, within an operating voltage range of the load switch, a single current limit is set based on the resistive value chosen by the designer. One disadvantage of using a single resistor for a wide operating voltage range (and thus a wide range of load current) is loss of accuracy. This loss may occur, because the resistive value and tolerance typically determine the level of granularity of current increments detectable.
For example, as shown in FIG. 1, as part of system design, a user selects a resistor having a resistive value, RSET. The user also defines a current limit to be associated with the resistive value, thereby establishing a one-to-one correspondence between the two parameters. The current limit is arbitrarily selectable by the designer. In this example, the designer has defined the current limit of 100 mA to correspond to a resistive value of 100Ω, and the current limit of 1 A to correspond to 1 kΩ. Within an operating voltage range of 0 to 1 V, the current limit cannot be set with the 1 kΩ resistor at a resolution below 1 A. For a resolution of the current limit below 1 A, a resistive value below 1 kΩ needs to be selected during system design. However, the operating voltage range must then also be limited accordingly. For example, in order to obtain a current limit of 140 mA, the resistive value needs to be decreased to 140Ω. One approach to changing the resistive value is to replace the resistor. Even with the different resistive value, the resolution may be low, such as due to resistor tolerance (e.g., 5%, 10% or more). When the resistor is replaced, the operating voltage changes as well. In this example, the operating voltage decreases to 0 to 0.14 V. In order to improve current limit control, more current limit detection with better resolution and accuracy is needed.
Therefore, there is a need for improved design of current limit detectors. One desired aspect of such design might be to substantially increase the accuracy and resolution obtainable using a particular resistive value.